Paper 2025/370

Simple Public Key Anamorphic Encryption and Signature using Multi-Message Extensions

Abstract

Anamorphic encryption (AE) considers secure communication in the presence of a powerful surveillant (typically called a ''dictator'') who only allows certain cryptographic primitives and knows all the secret keys in a system. The basic idea is that there is a second (anamorphic) mode of encryption that allows to transmit an anamorphic message using a double key to a receiver that can decrypt this message using a double key. From the point of view of the dictator the encryption keys as well as the ciphertexts in the regular and anamorphic mode are indistinguishable. The most recent works in this field consider public key anamorphic encryption (PKAE), i.e., the sender of an anamorphic message requires an encryption double key (or no key at all) and the receiver requires a decryption double key. Known constructions, however, either work only for schemes that are mostly of theoretical interest or come with conceptual limitations. In this paper we ask whether we can design such PKAE schemes without such limitations and being closer to PKE schemes used in practice. In fact, such schemes are more likely to be allowed by a cognizant dictator. Moreover, we initiate the study of identity-based anamorphic encryption (IBAE), as the IBE setting seems to be a natural choice for a dictator. For both PKAE and IBAE, we show how well-known IND-CPA and IND-CCA secure primitives can be extended by an anamorphic encryption channel. In contrast to previous work, we additionally consider CCA (rather than just CPA) security notions for the anamorphic channel and also build upon CPA (rather than just CCA) secure PKE. Finally, we ask whether it is possible to port the recent concept of anamorphic signatures, which considers constructing symmetric anamorphic channels in case only signature schemes are allowed by the dictator, to the asymmetric setting, which we denote by public-key anamorphic signatures (PKAS). Also here we consider security beyond IND-CPA for the anamorphic channel.